The manufacture of chemical pulp comprises two main phases, namely
a phase involving the cooking of lignocellulosic materials by means of chemical reagents intended to dissolve the greater part of the lignin and to release the cellulosic fibres, leading to an unbleached pulp,
a phase involving the delignification and bleaching of the unbleached pump, comprising in general several successive treatment stages interspersed in some cases with washing, dilution and/or concentration stages in order to achieve the residual lignin content and the whiteness that are desired.
The term chemical pulps will be taken to mean pulps which have undergone a delignification treatment in the presence of chemical reagents such as sodium sulphide in an alkaline medium (kraft or sulphate cooking) or else by other alkaline processes.
In recent years, numerous delignification and bleaching processes free from chlorine have been developed in addition to those which conventionally use chlorine and chlorine dioxide. Various kinds of delignification and bleaching agents are currently used for the treatment of the unbleached pulps. For example, it has been proposed that the chemical pulps be subjected to the action of oxygen in an alkaline medium, and then to delignification and bleaching treatments comprising treatments with ozone, peracids and hydrogen peroxide.
When chemical pulps are bleached with oxidizing agents such as ozone, peracids or hydrogen peroxide, it is advisable to remove from the pulp certain harmful metal ions. These metal ions having a harmful effect are ions of transition metals and include, among others, manganese, copper and iron, which catalyse decomposition reactions of the peroxidized reagents. They degrade the peroxidized reagents employed for the delignification and the bleaching via radical-type mechanisms and thus increase the consumption of these products while at the same time reducing the mechanical properties of the pulp.
Removal of the metal ions can be effected by a treatment with acid at the ambient temperature of the pulp. However, these treatments in an acid medium remove not only the harmful metal ions but also the ions of alkaline-earth metals such as magnesium and calcium, which have a stabilizing effect on the peroxidized reagents employed and a beneficial effect on the visual and mechanical qualities of the pulp.
It has been found recently that in chemical pulps, the metal ions are above all linked to carboxylic acid groups. Thus, the PCT patent application WO 96/12063 proposes a method for destroying selectively 4-deoxy-b-L-threo-hex-4-ene pyranosyluronic acid groups (hexene uronic groups) by treating the pulp at a temperature of between 85.degree. C. and 150.degree. C. and at a pH of between 2 and 5. The destruction of the hexene uronic groups reduces the kappa number from 2 to 9 units and reduces in a non-selective manner the adsorption of the ions of transition metals and alkaline-earth metals.
One of the major disadvantages of these processes in an acid medium is therefore that they are not selective with respect to certain metal ions, namely with respect to the harmful ions of transition metals.
A known means of selectively removing harmful metal ions from the pulp consists in the chelation of these ions. Unfortunately, this chelation stage requires a strict monitoring of the pH of the pulp often in a pH range which is close to neutral. Patent application EP 0 456 626 describes a pulp bleaching process in which a chelation stage (stage Q) is carried out in a pH range of between 3.1 and 9.0 before the treatment of the pulp with hydrogen peroxide (stage P). However, example 1 of this patent application shows that the maximum whiteness of the pulp after treatment with the peroxide comes to 66.1.degree. ISO and that this is achieved when the pH of stage Q is equal to 6.1. At higher pH values, the whiteness of the pulp declines rapidly and reaches not more than 51.9.degree. ISO at pH 7.7 and 56.4.degree. ISO at pH 9.1. It follows from this example that it is possible in theory to carry out a chelation stage in a broad pH range but that in practice the pH zone in which satisfactory results are obtained is very restricted and often close to neutral pH values where the buffer capacity of the pulp suspension is weak and in which the monitoring of the pH is difficult. As soon as the optimum pH value is departed from, in fact, the paper quality drops very sharply, so that the process requires strict monitoring of the pH. The pH optimum of the chelation depends on the pulp employed and lies for current chemical pulps in a pH range of between 4 and 7. However, each pulp has a specific optimum pH within this pH range of between 4 and 7 for stage Q. As soon as this optimum pH is departed from, the pulp quality obtained after treatment with hydrogen peroxide declines rapidly. In addition, the quantity of hydrogen peroxide consumed increases together with the production costs. In other words, even a small variation in the pH during stage Q has a considerable influence on the quality and/or the cost price of the chemical pulp. In industrial applications, it is difficult to monitor accurately the pH when the latter is close to neutrality, because the buffer capacity of the pulp suspension is relatively weak.
Furthermore, the known means of selectively removing harmful metal ions from the pulp, i.e. the chelation of these ions, requires the use of strong chelating agents. Patent application EP 0 456 626 describes a pulp bleaching process in which a chelation stage (stage Q) using aminocarboxylic chelating agents such as EDTA or DTPA is carried out in a pH range of between 3.1 and 9.0 before the treatment of the pulp with hydrogen peroxide (stage P).
One disadvantage of this process is linked to the use of very strong aminocarboxylic chelating agents such as ethylenediaminetatra-acetic acid (EDTA) of diethylenetriaminepenta-acetic acid (DTPA). As the pulp itself possesses sequestrating properties for ions of transition metals, it is in fact necessary to use appreciable quantities of aminocarboxylic chelating agents in order to remove these ions from the pulp. Moreover, it is necessary to use very strong aminocarboxylic chelating agents in order to remove these ions from the pulp. Other less powerful chelating agents have no effect on the ions which it is desired to remove. However, the use of aminocarboxylic chelating agents raises problems with regard to environmental protection. Since they are biodegradable to only a limited extent, they are difficult to destroye in conventional water treatment stations and some of them end up in rivers. These chelating agents can then solubilise heavy metals such as the mercury and cadmium contained in the sediments of these rivers and introduce them into the food chain.